The present invention relates to an automatic control device for the gearshift of industrial vehicles, such as loaders, excavators, lift trucks and the like, and in particular it relates to a device for controlling an automatic two-speed gearshift with coupling under torque for hydrostatic transmissions.
As is known, in order to move industrial vehicles equipped with an automatic gearshift of the described type, an endothermal motor actuates a differential axle for driving the vehicle by means of a hydrostatic transmission and of an automatic gearshift. The hydrostatic transmission generally comprises a variable-capacity hydrostatic pump connected to the output shaft of the endothermal motor and a variable-capacity hydrostatic motor which is actuated by the hydrostatic pump; the output shaft of said motor is connected to the differential axle by means of the gearshift, for example of the three-shaft type, comprising hydraulically actuated clutches so as to obtain two different speed ratios between the input shaft and the output shaft of said gearshift. The clutches (of the disk type) are actuated by means of an external constant-capacity hydraulic pump which feeds the pressurized oil intended to pack the disks of one of the two clutches and therefore engage one of the two gears. Feeding of pressurized oil to one clutch or the other (and therefore the engagement of one or the other of the gears) is caused by means of two solenoids controlled by an electronic actuation device appropriately connected to the operator's controls (allowing him to choose between manual or automatic operation of the gearshift and to control the direction of motion) and to the endothermal motor, to the hydrostatic transmission and to the gear control solenoids, so as to control the direction of rotation of the transmission's output shaft (and therefore the direction of motion) and generate appropriate energization and de-energization signals for the clutch control solenoids, in a per se known manner, according to the power of the motor and to the speed of the vehicle.
This automatic control system operates satisfactorily and is very reliable, but it is not free from disadvantages. When passing from the second gear to the first, in fact, it is not capable of ensuring a gradual speed reduction, and indeed it causes a sharp deceleration of the vehicle which is considered unpleasant and unacceptable by the vehicle's user.
Various solutions have been studied and tested in order to eliminate this problem, some of which entail complicated modifications of the electronic control part so as to actuate the solenoids (and therefore the respective clutches) only when the speeds of the disks of the first gear's clutch are identical. However, even these solutions have not solved the problem completely and have not eliminated the undesirable deceleration during gear shifting; said solutions furthermore entail considerable and expensive modifications of the elements of the transmission system, especially in the electronic control section.